2-Hydroxyarylimidazole-based colorimetric and ratiometric fluoride ion sensors

Abstract

Two novel 2-hydroxyarylimidazoles were developed as colorimetric and ratiometric sensors for the naked-eye detection of fluoride anions. The discriminating absorption and fluorescence responses to fluoride in acetonitrile as well as a methanol–acetonitrile solvent system were observed. The bathochromic optical changes are attributed to the deprotonated species evidenced from ¹H NMR titrations. Their selectivity for fluoride over acetate was successfully established in the methanol–acetonitrile mixture. A fluorene-bridged imidazole dimer, 4a demonstrated better response for fluoride than acetate in the neat acetonitrile. Moreover, the sensor 4a exhibited a larger ratio of fluorescence intensity response (I₄₈₉/I₄₃₁ = 51) than 4b (I₅₂₁/I₄₄₈ = 18) for fluoride in acetonitrile.

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Introduction

The development of chemosensors for anions and particularly fluoride ions has received immense attention in recent years. High levels of fluoride intake have been correlated with several health aberrations so it becomes more crucial to develop artificial sensors to detect the fluoride ions.¹ Generally, the neutral type sensors are comprised of hydrogen-bonding donors/acceptors to recognize anions, conversely the positively charged sensors interact with guest anions through electrostatic interactions.²

The colorimetric changes or emission quenching are the major event observed for the most of the fluoride sensors, only some of them exhibit emission intensity enhancement.³ A ratiometric sensor with efficient selectivity is necessary for a quantitative measurement.⁴ Although several fluorescent receptors for cations are reported, there is scarcity of reports of ratiometric fluorescent sensors for anions particularly for fluoride which necessitate its development.⁵ Substantial attempts have been established to formulate hydrogen-bonding donors/receptors to employ several types of signalling mechanisms to develop ratiometric fluorescent sensors for anions which includes fluorescence resonance energy transfer (FRET),⁶ intramolecular charge transfer (ICT),⁷ photoinduced electron transfer (PET),⁸ excimer/exciplex formation,⁹ metal-to-ligand charge transfer (MLCT)¹⁰ and proton transfer.¹¹

Proton transfer mechanism has been widely used to design anion sensors. In the presence of the anions, the hydrogen bonding between anion and the proton donors such as –NH/–OH or anion-induced deprotonation was observed, which resulted in large spectral changes.¹² Deprotonation is preferred by the highly acidic hydrogen-bond donors and strongly basic anions.¹³ But, in such proton transfer derived anion-sensing, if the basicity and the surface charge density of the anions are similar, the anion differentiation will be definitely poor, especially the spectral changes will not parallel the concentration of anion.

Various molecular structures based on pyrrole,¹⁴ diketopyrrolopyrrole,¹⁵ pyrazole,¹⁶ naphthalene diimide,¹⁷ maleimide,¹⁸ salicylaldehyde,¹⁹ acylhydrazone,²⁰ terphenyl,²¹ urea,^13b,22 thiourea,^22a,22b,23 benzoxazole,²⁴ thiazole,²⁵ hydroxyflavone,²⁶ silyl-protected phenol functionality²⁷ and Lewis acid²⁸ have been exploited for the fluoride detection. But excluding few exceptions, there is lack of selective probes. Particularly, proton transfer based-system gives similar spectral response for both of fluoride and acetate.

The simple organic compounds bearing suitable functional groups or heterocyclic rings that have binding sites can be realized as selective and effective ion receptor systems. The efforts towards structural modifications can develop such receptor systems which recognize ions selectively and act as a sensor. The imidazole-based receptors to recognize cations are known in literature but very few examples available about its anion-sensing.²⁹

The imidazole ring can act as an excellent hydrogen bond donor moiety in the anion-receptor systems, and the acidity of the –NH proton of the imidazole can be easily tuned by changing the electronic properties of the imidazole substituents which can converts imidazole derivatives into excellent anion sensors. The 2-(2-hydroxyaryl)imidazole derivatives are important class of excited state intramolecular proton transfer (ESIPT)³⁰ dyes and found applications in laser dyes and electroluminescent materials.³¹ However, the use of 2-(2-hydroxyaryl)imidazoles as a selective fluoride ion sensors is scarce. These derivatives can be easily deprotonated in the presence of appropriate basic ions resulting in the significant spectral changes to serve as optical sensor for such anions. The availability of two types of acidic protons (–NH and OH) will further improve their sensitivity. Despite all of these facts, no systematic studies on selective hydrogen-bonding-based probes of fluoride over acetate and their demonstration in particular solvent mixture have been undertaken. Taking advantage of fluorene which is well known for enhancing electro-optical properties once combined with other functionalities,³² we describe here the synthesis of fluorene-bridged 2-(2-hydroxyphenyl)imidazole (4a) and 2-(2-hydroxynaphthyl)imidazole (4b) receptors for the detection of fluoride.

Results and discussions

Synthesis and characterization

The synthetic method employed to obtain 2-(2-hydroxyaryl)imidazoles (4a and 4b) is shown in Scheme 1. The receptors were prepared in excellent yields by the condensation of tetraketone (3) with salicylaldehyde or 1-naphthaldehyde in the presence of ammonium acetate. The precursor 2,2′-(9,9-dibutyl-9H-fluorene-2,7-diyl)bis(1-phenylethane-1,2-dione) (3) was synthesized from corresponding 2,7-dibromo-9,9-dibutyl-9H-fluorene via Sonogashira coupling reaction³³ with phenyl acetylene followed by iodine catalyzed oxidation in DMSO.³⁴ The structures were characterized by standard spectroscopic methods.

Scheme 1 Synthesis of the receptors 4a and 4b.

Optical properties

The ability of receptors to recognize fluoride anion was performed by UV-vis and emission spectroscopic techniques by adding a standard solution of the tetrabutylammonium salt of anions in dry acetonitrile (neat) and methanol–acetonitrile solvent systems. The absorption spectra of the compounds 4a and 4b recorded in acetonitrile exhibited prominent peaks at 342 nm and 364 nm respectively which are attributable to π–π* electronic transitions of hydroxyaryl substituent and imidazole (Fig. 5(a) and 6(a)). Additionally, high energy bands were also observed originated from fluorene and imidazole localized electronic transitions. The hydroxynaphthyl derivative (4b) showed red-shifted absorption as well as emission than that observed for the hydroxyphenyl derivative (4a) because of extended conjugation.

The changes of UV-vis absorption of compounds upon addition of F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻ and AcO⁻ (20 equiv.) was carried out. On addition of fluoride or acetate ion to the receptors, a new peak at longer wavelength region was formed with decrease in parent absorption maxima while other anions (Cl⁻, Br⁻, I⁻ and HSO₄⁻) didn't cause noticeable change (Fig. S1 and S5†, Table 1). An apparent color change from colorless to light-yellow for 4a was observed in ambient light while 4b showed yellow to orange color change from colorless (Fig. 7 and 8). 4a evidenced better selectivity for F⁻ than AcO⁻ as it showed 13 nm more red-shifted absorption along with 9.2 × 10³ M⁻¹ cm⁻¹ higher extinction coefficient with F⁻ than that observed with AcO⁻.

Table 1 Photophysical data for 4a and 4b in the presence of F⁻ and AcO⁻

Dye	λabs, nm (ε × 10^3, M^−1 cm^−1)							λem, nm (ΦF, %)^a
	Acetonitrile			Methanol–acetonitrile				Acetonitrile			Methanol–acetonitrile
	Free	F^−	AcO^−	% v/v	Free	F^−	AcO^−	Free	F^−	AcO^−	% v/v	Free	F^−	AcO^−
a 2-Aminopyridine (Φ = 0.60 in 0.1 N H2SO4) as reference.
4a	342 (46.6)	404 (36.9)	391 (27.7)	1%	342 (46.6)	398 (34.4)	335 (45.0)	431 (56)	489 (35)	489 (20)	1%	431 (56)	492 (31)	434 (24)
4b	364 (57.0)	411 (52.3)	410 (50.5)	4%	364 (57.0)	406 (48.2)	370 (47.7)	448 (23)	521 (11)	521 (10)	4%	448 (23)	525 (7)	453 (10)

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Similarly the emission spectra of the compounds were also red-shifted with gradual decrease of free receptor peak on interaction with F⁻ and AcO⁻ ions (Fig. S9 and S13†, Table 1). Compound 4a was able to distinguish F⁻ and AcO⁻ spectrofluorimetrically as well, it gave 2-fold intense peak with fluoride than with acetate.

In order to find the optimum condition for the selective sensing of F⁻ ion over AcO⁻ we have analyzed the interaction of the dyes with anions in acetonitrile in the presence of different amounts of methanol. Protic solvents such as methanol has strong tendency to solvate the anions through hydrogen bonding and thus anion will not be readily available and thus receptors can't recognize them easily. In the presence of suitable amount of such protic solvent the anions can be differentiated depending upon their basicity. As acetate (pK_a, AcOH 4.75) is more basic than fluoride (pK_a, HF 3.2), it may not respond to receptors in methanol. According to different responses of the receptors towards F⁻ and AcO⁻ in various ratio of methanol–acetonitrile mixture, the appropriate proportion was demonstrated to recognize the F⁻ anion selectively (Fig. S2, S3, S6, S7, S10, S11, S14 and S15†). As hypothesized we found that 1% methanol is enough to selectively determine the presence of F⁻ by using 4a. Compound 4b required 4% methanol, such difference can be attributed to more acidic nature (phenolic –OH) of hydroxynaphthyl derivative, 4b than hydroxyphenyl analogue, 4a. In these solvent mixtures, absorption and emission experiments of receptors were accomplished by using different tetrabutylammonium anions (Fig. S4, S8, S12 and S16†). There was only ∼5 nm variations were observed in the absorption and emission of receptor + fluoride than that recorded in neat acetonitrile which is well discernible than with acetate. Representative plots of the absorbance ratio and emission intensity ratio (the ratio of new peak versus parent peak) of receptors upon addition of various anions in neat acetonitrile and methanol–acetonitrile mixture are shown in Fig. 1–4. In particular, there is less slump of absorbance and emission ratio was observed in the presence of fluoride over acetate in methanol–acetonitrile mixture. Significantly, 4a manifested a larger ratio of fluorescence intensity response (I₄₈₉/I₄₃₁ = 51) than 4b (I₅₂₁/I₄₄₈ = 18) for fluoride in neat acetonitrile.

Fig. 1 Plot of the absorbance ratio (A_{404 nm}/A_{342 nm}) of 4a after addition of different anions recorded in acetonitrile and 1% methanol–acetonitrile mixture.

Fig. 2 Plot of the emission intensity ratio (I_{489 nm}/I_{431 nm}) of 4a after addition of different anions recorded in acetonitrile and 1% methanol–acetonitrile mixture.

Fig. 3 Plot of the absorbance ratio (A_411nm/A_364nm) of 4b after addition of different anions recorded in acetonitrile and 4% methanol–acetonitrile mixture.

Fig. 4 Plot of the absorbance ratio (I_{521 nm}/I_{448 nm}) of 4b after addition of different anions recorded in acetonitrile and 4% methanol–acetonitrile mixture.

Finally, titrations were performed for the receptors by using tetrabutylammonium fluoride for further investigation. Fig. 5 and 6 shows the absorbance and fluorescence changes for 4a and 4b on addition of different amounts of F⁻ in acetonitrile. With the progressive addition of F⁻ to 4a, the absorbance at 342 nm decreased with emergence of a new peak at 404 nm. An isobestic point was noticed at 377 nm (Fig. 5(b)). A new peak at 411 nm with decrease at 364 nm having an isobestic point 384 nm was observed for the compound 4b (Fig. 6(b)). In the emission spectra, the peak at 431 nm was displaced by a longer wavelength peak at 489 nm for 4a while 4b exhibited peak intensity drop-off at 448 nm with rise of new peak at 521 nm. The optical response can be assigned to the deprotonation of the compounds (Fig. 12) by these anions which induced intramolecular charge transfer (ICT) in the host–guest complex. The photoluminescence quantum yield was found to decrease after addition of both fluoride and acetate also ensures the existence of charge transfer species which was stabilized in the excited state.

Fig. 5 (a) Absorption (2 × 10⁻⁵ M) and (b) emission (2 × 10⁻⁶ M) spectral changes during the progressive addition of F⁻ to 4a recorded in acetonitrile; plot of absorbance at 404 nm and emission intensity at 489 nm for 4a versus F⁻ concentration recorded in acetonitrile (inset).

Fig. 6 (a) Absorption (2 × 10⁻⁵ M) and (b) emission (2 × 10⁻⁶ M) spectral changes during the progressive addition of F⁻ to 4b recorded in acetonitrile; plot of absorbance at 411 nm and emission intensity at 521 nm for 4b versus F⁻ concentration recorded in acetonitrile (inset).

Fig. 7 Images of colorimetric response (top) and under UV-illumination (bottom) of 4a in acetonitrile (left) and methanol–acetonitrile (1% v/v) (right) (from L → R: free 4a, with F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻ and AcO⁻).

Fig. 8 Images of colorimetric response (top) and under UV-illumination (bottom) of 4b in acetonitrile (left) and methanol–acetonitrile (4% v/v) (right) (from L → R: free 4b, with F⁻, Cl⁻, Br⁻, I⁻, HSO₄⁻ and AcO⁻).

Plots of absorbance at 404 nm and emission intensity at 489 nm versus fluoride ion concentration for 4a are shown in Fig. 5(b) (inset). A linear correlation was obtained which established their potential utility to detect fluoride ion ratiometrically. Similar correlations were observed for 4b also (Fig. 6(b)).

In order to analyse the sensitivity for fluoride, the limit of detection (LOD) was calculated in the absorption as well as in emission (see ESI†). From the absorption experiments, the LOD of 4a and 4b for fluoride is determined to be 0.049 μM and 0.042 μM while emission experiments provides 0.030 μM and 0.041 μM respectively. The value is highly significant and much lower than the standard level of fluoride in environment and thus arrogate the sensitivity of the receptors.

Determination of the stoichiometry and association constants (K_a)

The stoichiometry of the complexes formed between the receptors and anions was determined by Job's plot.³⁵ In this method, the total molar concentration of receptor and anion are held constant, but their mole fractions are varied. A measurable parameter that is proportional to complex formation (such as absorption signal) is plotted against the mole fractions of these two components (Fig. S17–S20†). The maximum on the plot corresponds to the stoichiometry of the two species. Fluoride preferred a 1 : 4 association while the acetate formed 1 : 2 complexes. As illustrated in Fig. 12, two fluoride ions are required to effectively interact with the –NH and –OH hydrogens but a single acetate anion can engage them in a bidentate fashion. On the basis of these results we can suggest the mode of different binding of fluoride and acetate with the receptors. Both of the isomeric enol-form (I and II) of the compounds will allow the fluoride binding while only E_anti rotamer (II) will favor such binding of acetate.

The interaction of fluoride anion with the compounds was further investigated by the calculation of association constant K_a. The complex formation between a proton donor (LH) and fluoride anion is characterized by the following equilibrium:

LH + nF⁻ ⇌ [LH⋯F_n⁻] ⇌ L⁻ + nHF

The association constant K_a can be expressed as

where [LH⋯F_n⁻] is the concentration of complex, [LH] is the concentration of free receptor (proton donor), [F⁻] is the concentration of fluoride anion and n is the number of equivalent of [F⁻] required for the complex formation with proton donor.

The association constant K_a can be calculated by absorption spectral changes occurred during addition of fluoride ion into the solution of receptors by using the Scott equation.³⁶ The equation for the 1 : 4 complex formation is given by

where ΔA_obs is the change in absorbance upon addition of fluoride, ΔA_c is the absorption change between pure complex and the free component at the saturation, and [G] is the concentration of the guest. A plot of [G]⁴/ΔA_obs versus [G]⁴ will give a slope = 1/ΔA_c and intercept = 1/ΔA_cK_a. The ratio slope/intercept will give the association constant.

The plots gave good linear fit with correlation coefficient of >0.99, which further confirm the 1 : 4 stoichiometry (Fig. 9). The larger numeric value of K_a predicts the higher sensitivity of receptor towards anion/guest. The receptor 4b (K_a = 8.52 × 10¹⁷) showed higher sensitivity than 4a (K_a = 5.04 × 10¹⁷) attributable to the stronger acidity of hydroxynaphthyl unit.

Fig. 9 Scott plots for (a) 4a + F⁻ and (b) 4b + F⁻.

¹H NMR titrations and mechanism

To gain structural information about the receptor–fluoride complexes the ¹H NMR titrations were performed. The ¹H NMR spectra of the 4a and 4b in the presence of one and ten equivalents of tetrabutylammonium fluoride in CD₃CN were recorded and compared with that of free receptor (Fig. 10 and 11). On addition of one equivalent of fluoride anion, the peaks attributable to –OH and –NH protons disappeared, which confirms the deprotonation of the receptors. As the receptors contain two equivalent sets of –NH and –OH units, four equivalents of fluoride ions are required to deprotonate them completely. The disappearance of –NH and –OH signals on addition of one equivalent of fluoride ion may be ascribed to the fluoride assisted rapid proton exchange occurring besides deprotonation (Fig. 12).

Fig. 10 ¹H NMR titration of 4a on addition of F⁻ recorded in CD₃CN.

Fig. 11 ¹H NMR titration of 4b on addition of F⁻ recorded in CD₃CN.

Fig. 12 Proposed mechanism for the formation of anionic species as the outcome of F⁻ interaction with the receptors.

In the absence of fluoride ion most of the aromatic proton signals were observed as broad signals which were resolved on addition of fluoride ion. Broadened signals observed for the free receptors originate probably due to the rapid exchange of protons between the nitrogen and oxygen atoms. But introduction of fluoride probably arrests this exchange. Fluoride induced deprotonation increases the electron density through bond propagation and cause upfield signals.^14a The addition of ten equivalents of fluoride ensured complete deprotonation and the charge delocalization in the aromatics resulted in further upfield shift. The phenolic proton, “f” (proximal toward –NH of imidazole) of 4a, first shifted downfield on addition of one equivalent of fluoride ion while the progression of fluoride concentration to ten equivalents produced an upfield shift. These may be due to initial C–H bond polarization occurring through-space (downfield shift) which may be no longer operating at higher concentrations of fluoride ion.^14a,37 It is noteworthy that the “g” proton (hydroxynaphthyl proton proximal to –NH imidazole) of 4b gave the same result as of 4a on addition of 1 equivalent of fluoride but they further moved to downfield on excess addition of fluoride. Moreover, these proximal protons showed broad peak at the 1 equivalent fluoride-addition stage.

In consistent with these observations it is proposed that the enol forms of the dyes I and II on interaction with one equivalent of fluoride will produce the phenolate anion III (Fig. 12). Further addition of fluoride ion would facilitate the deprotonation of –NH (imidazole), to form dianion IV which will rapidly convert to the more stable anti form V. This will be in equilibrium with the keto form VI. These dianionic keto forms are responsible for charge transfer transition and become the basis for the absorption and emission changes. In ¹H NMR, the proton signals belong to the C-4 and C-5 substituents of imidazole become resolved and symmetrical on addition of ten equivalent of fluoride addition which support the possible existence of dianion VI. The appearance of C=O band (1662 cm⁻¹) in the IR of receptors + F⁻ further evidenced the involvement of VI (Fig. S21 and S22†).

Conclusions

In summary, we have developed two novel 2-(2-hydroxyaryl)imidazole-based highly efficient colorimetric and fluorescent fluoride sensors. The receptors were found to function as colorimetric and ratiometric fluorescent chemosensor for fluoride. In methanol–acetonitrile mixture the dyes were demonstrated to selectively detect the presence of fluoride ions. The receptor 4a showed better selectivity for fluoride over acetate in the neat acetonitrile itself as observed from absorption and emission spectroscopy. The ¹H NMR titrations revealed the formation of anionic species which induced intramolecular charge transfer in the molecule and red shifted absorption and emission. The larger ratio of fluorescence intensity response was also exhibited by sensor 4a (I₄₈₉/I₄₃₁ = 51) than 4b (I₅₂₁/I₄₄₈ = 18) in neat acetonitrile which further validate its practical application.

Experimental

Materials and methods

Chemicals were purchased from Aldrich, sd fine-chem, Spectrochem or Thomas-Baker and used as received. Solvents were dried and distilled immediately prior to use by standard procedure. Column chromatography purification was performed with the use of silica gel (230–400 mesh, Rankem) as the stationary phase in a column with 40 cm long and 3.0 cm diameter. ¹H NMR and ¹³C NMR were recorded in CDCl₃ CD₃CN or DMSO-d₆ on a Bruker AV 500 O FT-NMR spectrometer operating at 500.13 and 125.77 MHz, respectively. High resolution mass spectrometric measurements were carried out using ESI mass spectrometer. The IR spectra were recorded on NEXUS FT-IR (THERMONICOLET). UV-Vis spectra were recorded at room temperature in quartz cuvettes using UV-1800 Shimadzu spectrophotometer. Fluorescence measurements were carried out on RF-5301 PC Shimadzu spectrofluorophotometer. Quantum yield of the dyes were calculated by following standard procedure and using 2-aminopyridine (Φ_F = 0.60 in 0.1 H₂SO₄)³⁸ as reference. Corrections due to dye absorption and refractive indices of the solvents used for measurement were incorporated in the calculation.

UV-vis and fluorescence titrations were performed on 2 × 10⁻⁵ M solutions of compounds in spectroscopic grade acetonitrile. Aliquots of freshly prepared standard solutions of tetrabutylammonium salt of anions were added and subsequently corresponding UV-vis and fluorescence spectra were recorded. Similarly for methanol-titrations, to a solution of compounds + acetate/fluoride, 10 μL of methanol was added in an incremental fashion. Upon each addition, the solution was stirred for 2 min to reach equilibrium.

For ¹H NMR titrations, 5 × 10⁻³ M solutions of compounds were prepared in CD₃CN and 0.5 mL portion was transferred to a NMR tube. A small aliquot of Bu₄F in CD₃CN was introduced and their corresponding spectra were recorded.

Synthesis

2,7-Dibromo-9,9-dibutyl-9H-fluorene (1)³⁹ can be synthesized according to general reported procedure.

9,9-Dibutyl-2,7-bis(phenylethynyl)-9H-fluorene (2)

A mixture of 2,7-dibromo-9,9-dibutyl-9H-fluorene (1) (2.18 g, 5 mmol), phenyl acetylene (1.12 g, 11 mmol), Pd(PPh₃)₂Cl₂ (70 mg, 0.1 mmol), PPh₃ (52 mg, 0.2 mmol) and CuI (20 mg, 0.1 mmol) in 50 mL of triethylamine was degassed with nitrogen. The mixture was stirred and refluxed under nitrogen for 12 h, then cooled to room temperature. It was poured into water and extracted with ethyl acetate. The organic layer was washed with brine for several times, separated and dried over anhydrous Na₂SO₄. The solvent was removed and crude product was purified by column chromatography on silica gel (hexane–dichloromethane). Yellow solid; yield 91%; mp 150–152 °C; IR (KBr, cm⁻¹) 2204 (ν_C≡C); ¹H NMR (CDCl₃, 500.13 MHz) δ 7.68 (d, J = 7.5 Hz, 2H), 7.52–7.59 (m, 8H), 7.35–7.39 (m, 6H), 1.99–2.02 (m, 4H), 1.07–1.14 (m, 4H), 0.69 (t, J = 7.5 Hz, 6H), 0.57–0.64 (m, 4H); ¹³C NMR (CDCl₃, 125.77 MHz) δ 151.1, 140.7, 131.6, 130.8, 128.4, 128.3, 126.0, 123.4, 122.0, 120.0, 90.4, 89.8, 55.2, 40.3, 25.9, 23.1, 13.9; HRMS (ESI) m/z calcd for C₃₇H₃₅ (M + H) 479.2733, found 479.2739.

2,2′-(9,9-Dibutyl-9H-fluorene-2,7-diyl)bis(1-phenylethane-1,2-dione) (3)

A mixture of 9,9-dibutyl-2,7-bis(phenylethynyl)-9H-fluorene (2) (1.20 g, 2.5 mmol) and iodine (1.27 g, 5 mmol) was dissolved in 20 mL of DMSO. The mixture was heated at 140 °C for 12 h and then treated with 10% aq. sodium thiosulfate solution followed by extraction with ethyl acetate. The extract was dried over anhydrous Na₂SO₄. The solvent was evaporated and residue was chromatographed on a silica gel column (hexane–dichloromethane) to collect pure product. Yellow solid; yield 88%; mp 118 °C; IR (KBr, cm⁻¹) 1671, 1594 (ν_C=O); ¹H NMR (CDCl₃, 500.13 MHz) δ 8.08 (d, J = 1.0 Hz, 2H), 8.02–8.04 (m, 4H), 7.92 (dd, J = 8.0 Hz, 1.5 Hz, 2H), 7.86 (d, J = 8.0 Hz, 2H), 7.67–7.70 (m, 2H), 7.53–7.56 (m, 4H), 2.03–2.06 (m, 4H), 1.06 (sext, J = 7.5 Hz, 4H), 0.65 (t, J = 7.5 Hz, 6H), 0.51–0.58 (m, 4H); ¹³C NMR (CDCl₃, 125.77 MHz) δ 194.6, 194.3, 153.0, 145.8, 135.0, 133.11, 133.08, 130.4, 130.0, 129.1, 123.8, 121.4, 55.9, 39.6, 26.0, 22.8, 13.7; HRMS (ESI) m/z calcd for C₃₇H₃₅O₄ (M + H) 543.2530, found 543.2534.

2,2′-(5,5′-(9,9-Dibutyl-9H-fluorene-2,7-diyl)bis(4-phenyl-1H-imidazole-5,2-diyl))diphenol (4a)

A mixture of 2,2′-(9,9-dibutyl-9H-fluorene-2,7-diyl)bis(1-phenylethane-1,2-dione) (3) (0.54 g, 1 mmol) salicylaldehyde (0.24 g, 2 mmol) and ammonium acetate (0.77 g, 10 mmol) in 10 mL of acetic acid was refluxed for 24 h. The reaction mixture was cooled and poured in to water to precipitate solid. The solid was washed thoroughly with water, dried and purified by column chromatography on silica gel by using dichloromethane–ethyl acetate mixture as eluent. Light-yellow solid; yield 85%; mp 124–126 °C; IR (KBr, cm⁻¹) 3423; ¹H NMR (DMSO-d₆, 500.13 MHz) δ 13.02–13.10 (m, 4H), 8.06 (d, J = 7.5 Hz, 2H), 7.75–7.97 (m, 2H), 7.55–7.62 (m, 6H), 7.36–7.38 (m, 5H), 7.27–7.30 (m, 5H), 6.99 (d, J = 8.5 Hz, 2H), 6.96 (t, J = 7.5 Hz, 2H), 1.67–1.75 (m, 4H), 0.99 (sext, J = 7.5 Hz, 4H), 0.63 (t, J = 7.5 Hz, 6H), 0.48 (s, 4H); ¹³C NMR (CDCl₃, 125.77 MHz) δ 157.5, 145.8, 130.6, 128.7, 127.8, 126.6, 123.3, 120.2, 119.0, 117.8, 112.6, 55.2, 40.1, 26.1, 23.1, 14.0; HRMS (ESI) m/z calcd for C₅₁H₄₇N₄O₂ (M + H) 747.3694, found 747.3695.

1,1′-(5,5′-(9,9-Dibutyl-9H-fluorene-2,7-diyl)bis(4-phenyl-1H-imidazole-5,2-diyl))dinaphthalen-2-ol (4b)

Following the general procedure for synthesis of 4a by using a mixture of 3 (0.54 g, 1 mmol), 2-hydroxy-1-naphthaldehyde (0.34 g, 2 mmol) and ammonium acetate (0.77 g, 10 mmol) in 10 mL of acetic acid. Yellow solid; yield 75%; mp 120–122 °C; IR (KBr, cm⁻¹) 3435; ¹H NMR (DMSO-d₆, 500.13 MHz) δ 8.15 (d, J = 8.5 Hz, 2H), 7.91 (d, J = 9.0 Hz, 2H), 7.85–7.89 (m, 4H), 7.59–7.63 (m, 6H), 7.48–7.52 (m, 4H), 7.32–7.38 (m, 10H), 1.74–1.77 (m, 4H), 1.03 (sext, J = 7.5 Hz, 4H), 0.65 (t, J = 7.5 Hz, 6H), 0.51–0.57 (m, 4H); ¹³C NMR (CDCl₃, 125.77 MHz) δ 156.8, 151.5, 145.2, 140.6, 131.4, 130.5, 129.6, 128.8, 128.7, 128.0, 127.9, 126.8, 123.2, 122.7, 121.9, 120.4, 119.6, 105.6, 55.3, 40.2, 26.2, 23.3, 14.2; HRMS (ESI) m/z calcd for C₅₉H₅₁N₄O₂ (M + H) 847.4007, found 847.4008.

Acknowledgements

Financial support from Council of Scientific and Industrial Research, New Delhi, India to KRJT is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR spectra of the newly synthesized compounds, titration curves in methanol–acetonitrile, Job's plot, IR spectra, and detection limit calculations are provided in the supplementary information. See DOI: 10.1039/c4ra10482j

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